“Both John the Baptist and archangel Gabriel are both important figures who practiced the religion.” How is it known? From the Britannica site. “The Mandaeans viewed Jesus as a false messiah but revered John the Baptist, who performed miracles of healing through baptism, which the Mandaeans viewed as a magical process giving immortality, purification, and physical health.”
That they later adopted the individuals as their own is different from saying those two were of that religion.